Helix coupling arrangements



May 17, 1955 c. F. QUATE HELIX COUPLING ARRANGEMENTS Filed June 12, 1952INVENTOR C. F. QUA TE ATTORNEY United States Patent 2,708,727 HELIXCOUPLING GEMENTS Calvin F. Quate, Berkeley Heights, N. J., assignor toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application June 12, 1952, Serial No. 2%,185 .12Claims. (Cl. SIS-3.5)

This invention relates to microwave transducers, and more particularlyto arrangements for energy transfers between helical wave guidingstructures, for example, of the kind characteristic of helix-typetraveling Wave tubes, and alternative forms of wave guiding structures,such as hollow wave guides and coaxial lines.

For purposes of illustration, it will be convenient to discuss thepresent invention with particular reference to energy transfers to andfrom the wave guiding structures of helix-type traveling wave tubes,although the applicability of the principles of the present invention toanalogous arrangements is not intended to be precludedthereby. In suchtraveling wave tubes, a wave circuit comprising an'elongatedhelical'conductor propagates radio frequency signal waves therethroughat axial velocities slower than the velocity of light and an electronstream is projected in the direction of wave propagation through theelectric field set up by the wave circuit. By proper adjustment of therelative velocities of the propagated wave and the electron stream, thewave and the stream can be made to interact in a cumulative fashionwhereby amplification of the wave {is realized. In such tubes, it isimportant to secure over the broad frequency range in which the tubeoperates accurate impedance matches between the helix wave circuit andthe signal input 'and output cir cuits which are coupled in energyexchange relationship thereto for introducing input signals and derivingamplified outputs, respectively. If mismatches are here present,components of the radio frequency waves tend to be reflected back andforth along the helix wave circuit. Such components generally make forsignal degradation and, in some cases, may result in tube instability.Additionally, for efliciency of operation, it is important that thecoupling provided between the helix circuit and the input and outputcircuits be 'high and substantially :uniform over the entire operatingfrequency range.

An object of the present invention is to facilitate the energy transferto and from wave circuits which comprise helical conductors.

A related object is to improve the impedance match in helix-typetraveling wave tubes between the helix wave circuit and the signal inputand output circuits which are coupled in energy exchange relationtherewith.

In the copending application Serial No. 270,721, filed on February 8,'1952,for J. R. Pierce, there are described arrangements for couplinghelix circuits to other suitable forms of Wave transmission circuits.'In each :of these arrangements, theelectric fieldcoupling between thehelix and such other transmission circuit is confined to a short portionof the helix and an impedance transformer is utilized for reducing thehelix impedance at the short coupling portion. The present inventionrelates to modifications which facilitate the design and fabrication ofsuch coupling arrangements in traveling wave tubes.

In particular, it is advantageous to provide a supporting structurewhich maintains the helix rigid and in accurate alignment and alsopermits the positioning of the impedance transformer sufiiciently closeto the helix.

- follows the coupling portion thereof, and a r gular wave guides 13 and14 are 2,708,727 t n ed May 195.5

2 The present invention is directed toward such a structure andparticularly to one which additionally serves to support Whatever fieldconfining elements there maybe desired to supplement the action of theimpedance transformer as described in the above-mentioned Pierceapplication.

In accordance with the present invention, the helix is supported infield coupling relation along a short coupling portion with another formof wave transmission circuit, for example, a hollow wave guide, by meansof a plurality of dielectric rods which are spaced apart and extendlongitudinally in contact with the helix to furnish Support therefor.Each of these rods is notched or slotted along a region thereof adjacentthe portion of the helix which conductive member is passed through thesenotches or slots and around the helix for transforming the heliximpedance at the coupling portion. In this way, there is achieved arugged structure which is relatively easy to construct and assemble in atraveling wave tube.

The invention will be better understood from the following more detaileddescription taken in conjunction with the accompanying drawings inwhich:

Fig. 1 shows in longitudinal cross section a typical helix typetraveling wave tube wherein is embodied an illustrative couplingarrangement in accordance withthe invention;

Fig. '2 is a cross sectional view taken along the line 2*2 of Fig. lwhich illustrates the relative positioning of the various elements ofthe coupling arrangement;

Fig. 3 represents an enlargement of the coupling .arrangement of thetube shown in Fig. l; and

Fig. 4 shows an illustrative arrangement for coupling a helix to acoaxial line in accordance with the invention.

For purposes of exposition, it has been thought undesirable to draw thevarious figures to scale, since it has been found advantageous toexaggerate the size of some of the'elements which figure mostprominently in the coupling arrangement which is the basis of thisinvention.

With reference now to the drawings, the helix type traveling wave tube"10 shown by wayof example inFig, l is constructed primarily of anon-magnetic material, which, for increased ruggedness and to minimizeneed of shielding, can be a metal -(e. g. copper). in general, to avoiddistortion of the strong magnetic field used to collimate and focus theelectron beam, it is advantageous that the use of magnetic materials inthe tube structure be kept at a minimum. Envelope 11 includes, asintegral parts thereof, a cylindrical elongated centralchamber 12,signal input and output circuit connections which comprise the hollowwave guides 13 and 14, respectively, an electron gun housing 15, and atarget housing 16. Substantially coaxial with this chamber 12 andextending at its two ends into the input and output wave guides '13 and14, which are suitably apertured therefor, there is disposed the helix17 Which serves .as the slow wave circuit along which electromagneticwaves are transmitted between the input and output connections. The:helix is supported by a series of dielectric ceramic rods 18 which areSpaced apart around the periphery of .the helix and extend parallel tothe helix axis in contact with the outer surface of the helix along thelength thereof. The walls of the guides can be notched for maintainingthe rods rigidly in position therebetween. The input and output rectanofconventional rectangular cross section and their long dimensions arenormal to the plane of the drawing. In this way, when transverseelectric waves are propagating in .the wave guides 13 and =14, the axisof the helix .17 will be parallel to the electric fields within the waveguides.

An electron gun 19 situated inthe electron gun housing 15 and acollector electrode 34 in the target housing 16 in target relationshipwith the electron gun serve as the termini of the electron stream. Theelectron gun is of a standard design and comprises a heater compartment31, of which the face nearest the central chamber 12 includes a circulardisk 32 which is perpendicular to and concentric with a continuation ofthe axis of the helix. This disk is activated and serves as a cathodewhen heated. A heating coil 33 is provided within the compartment 31 andis supplied with heating current from a suitable potential source by wayof leads (not shown) brought out through a glass vacuum seal.

At the opposite end of the tube envelope within the housing 16, there ispositioned the target electrode 34 which, generally though notnecessarily, is maintained at a positive potential with respect to thecathode 32 by means of suitable potentials applied by way of leads (notshown) brought out through a glass seal. When heated, the cathode 32emits electrons which are directed through the input wave guide 13 whichis suitably apertured for their passage therethrough, down the centralchamber 12 flowing close past the helix 17, and through the output waveguide 14, also suitably apertured, to the collector 34. Along this pathand perpendicular thereto, there is first disposed the field shapingelectrode 35 which comprises an annular disk through whose orifice theelectron stream passes. Further along the electron path is positioned acontrol electrode 36 which is also an apertured disk through which theelectrons flow. The potential on this grid can be varied to control theintensity of the electron stream. Still further along the path is theaccelerating electrode 37 which also is annular for the flow ofelectrons therethrough. Its potential is maintained positive withrespect to the cathode by a suitable voltage source. In practice, it isgenerally convenient to maintain this electrode, the tube envelope andthe helix at ground potential and, accordingly, the cathode 32 at apotential negative to ground. The accelerating potentials are chosen toimpart an electron velocity substantially equal to the axial wavevelocity along the helix. A magnetic coil (not shown) is disposed aroundthe exterior of the tube for producing a longitudinal magnetic fieldparallel to the axis of the helix for reducing radial components ofelectron velocity and focussing the stream into a well definedcylindrical beam, preferably hellow, coaxial with the helix.

In order to keep the interior of the tube envelope evacuated, the waveguides 13 and 14 are sealed off with cover glasses 51 andr52,respectively, which are made vacuum tight, and the central chamber isprovided with an exhausting tubulation which however has not been shownhere.

The description hitherto has been of a typical helix traveling wave tubeand many variations thereof are equally suitable for the purposes of theinvention. In'particular the tube envelope can be of a glass or similarnon-conductive material, in which case the elongated central portionthereof is supported between two suitably apertured wave guides so thatthe helix ends extend into the wave guides for electromagnetic couplingthereto in accordance with principles familiar to workers in this art.

As has been indicated above, the' invention is directed to improving thematch between the helix and the wave guides to which it is coupled. Asis described in the copending Pierce application to which reference hasbeen made above, it is found advantageous in coupling a helix to a waveguide to confine the coupling therebetween to a short end portion of thehelix, generally less than half a Wavelength long, and to provide animpedance transformation for reducing the helix impedance at thiscoupling region.

With particular reference now to the arrangement for coupling the helix17 to the input wave guide, which is shown in an enlarged view in Fig. 3and in cross section in Fig. 2, the end 41 of the helix extends into thewave guide and is connected to the cylindrical conductive post 42 whichextends from the guide wall 13A.

It is advantageous that the pitch of the helix remain uniform up to thepoint of connection. To this end, a portion of the end of the conductivepost 42 is beveled in a pitch corresponding to that desired for thehelix and the end 41 of the helix is fitted flush with this beveledportion and the connection is made there. As a result, there is effectedbetween the conductive post 42 and the helix 17 a smooth transition, andmore importantly, one that can be reproduced quite exactly in aconvenient fashion. Alternatively, the helix end can be integral with acylinder formed by brazing together a plurality of closely spaced turnsof the conductor. The field coupling is confined to a suitably shortportion 43 of helix by means of an apertured conductive member 44 whichsurrounds the intermediate portion of the helix in the wave guide,leaving exposed to the wave guide field only the coupling portion 43 anda somewhat longer end portion 45. This longer I exposed portion 45 isunimportant from a coupling standpoint, being made too long for the waveguide field to have an appreciable etfect. The conductive member 44 issuitably positioned and dimensioned so that the energy inci' dentthereon is used to propagate a wave along the helix in the direction ofelectron flow. Additionally, to secure the impedance transforming effectdesired, there is provi cled a hollow cylindrical or ring member 49, forforming a quarter wave impedance transformed. The general principles ofsuch quarter wave impedance transformers are more fully described onpage 188 in the Radio Engineers Handbook by F. E. Terman, published byMcGraw-Hill Book Company, New York (1943). In practice, it has beenfound desirable to position this transformer section 49 to surround asclosely as possible the quarter wavelength helix portion 46 followingthe coupling portion 43. To this end, each of the dielectric rods 18which serves to support the helix is cut away to form a slot or notch 47along its inner surface adjacent the helix portion, and the member 49 ispassed therethrough around but spaced apart from the helix portion.Moreover, it has been found convenient to support the conductive member44 around the dielectric rods 18, in contact with portions of the ringmember 49 so that the two may be maintained at the same potential level.A longitudinal slot 57 is provided on the inner surface of the outerring member 44 to accommodate each of the four rods 18 as shown in thetransverse cross-sectional view of Fig. 2, so that in the longitudinalcross-sectional planes shown in Figs. 1 and 3 the members 44 and 49 arenot shown to be in contact. As clearly appears in Fig. 2, however,members 44 and 49 are in contact at all points not included in the fourslots 57.

At the output end of the tube shown in Fig. 1, the helix 17 is coupledto the wave guide 14 in an analogous fash: ion, so that for the sake ofbrevity, a description thereof is being omitted.

It is, of course, possible to utilize a coupling arrangement of the kinddescribed at only one of the two ends of the helix circuit inconjunction with some other suitable form of coupling arrangement at theother end.

Moreover, the basic arrangement described can be modified in variousways. It may be desirable in special instances to eliminate the fieldconfining member 44' and employ only the impedance transformer member49. Alternatively, the field confining member 44 can take a variety offorms. For example, it may be desirable in other instances to extend thefield confining member from the wave guide wall 13B, in which case itmay also be desirable to vary the geometry of this end portion of thewave guide for example, to decrease in this region this short dimensionof the wave guide for forming a flat end portion.

These principles described can be applied to arrangements for coupling ahelix to a coaxial line form of wave transmission circuit. By way ofexample Fig. 4 shows an arrangement in which the helical conductor;17 isin an energy exchange relation with the coaxial terminal 62 whichcomprises an inner cylindrical conductor 63 enclosed by a hollow outercylindrical conductor 64. The inner conductor 63 can be solid or, as isadvantageous for the traveling wave tube application shown, tubular forpassage therethrough of an electron stream. In such application theouter conductor 64 c n be integral with the envelope of the tube. Thehelix is maintained coaxial with the inner and outer conductors by meansof a series of dielectric support rods 18 extending longitudinally alongthe surface of the helix and spaced apart therearound. One end 41 of thehelix is connected'to an end of the inner conductor 63, preferably inthe manner described aboveto provide a smooth easily reproducedtransition from the inner conductor 63 to the helix. The field couplingregion between the helix and the coaxial line is formed by extending aprojection or base member 65 from the outer conductor 64 towards theinner conductor for forming a gap therebetween in which the elec- I tricfield resulting from a wave propagating along the coaxial line has alarge component in the direction of the helix axis. This axial componentis reinforcedand stray capacitance reduced by having the projectionextend obliquely as shown towards the inner conductor. The coupling gapbetween the inner and outer conductors is adjusted so that the couplingportion 43 of helix in this gap is less than one half a wavelength.Additionally, as above, an annular conductive member 49 surrounds thequarter wavelength of helix 46 following the coupling 43 and each of thesupport rods has that portion of its inner surface opposite this quarterwavelength of helix notched in a slot 47 for passage therethrough of thering member 49 which serves as the impedance transformer.

It is to be understood that the above-described embodiments are merelyillustrative of the general principles of the invention. In particularvarious other modifications of the arrangements shown in theabove-identified Pierce application, can be devised by a worker in theart without departing from the spirit and scope of the invention.

What is claimed is:

1. In an electronic device, input and output wave guiding circuits, ahelix wave transmission circuit extending between the input and outputwave guiding circuits, one end region thereof in field coupling relationwith the input circuit and the other end region thereof in fieldcoupling relation with the output circuit, a plurality of dielectricsupport members extending longitudinally along the surface of said helixand spaced apart around the helix periphery, each of said supportmembers slotted along two portions of its inner surface adjacent toportions of the helix following the two coupling end regions, and meansat each coupling region for matching the helix impedance to theassociated wave guiding circuit comprising a conductive ring memberclosely surrounding but spaced apart from the helix and passing throughthe slots in the support members.

2. In an electronic device, means forming and defining a path ofelectron flow, a helix along said path for guiding electromagnetic wavesfor interaction with the electron fiow, a wave transmission circuit inenergy exchange relation with an end portion of said helix, a pluralityof dielectric rods extending longitudinally along said path and spacedapart around the periphery of said helix for supporting the helix alongsaid path, each of said dielectric rods being notched along a portion ofits surface adjacent a portion of helix following said end portion ofthe helix, and a conductive ring member passing through the notchesclosely surrounding but spaced apart from the helix.

3. An electronic device according to claim 2 which includes a fieldconfining member surrounding a portion of the helix following the endportion and passing around the dielectric rods, which is at thepotential of the conductive ring member.

4. In an electronic device, input and output hollow wave guides, a helixwave transmission circuit having one end extending in the input waveguide for coupling thereto and the other end extending in the outputwave guide for coupling thereto, a plurality of dielectric rodsextending along the outer surface of said helix spaced apart around thehelix periphery for supporting the helix between the two wave guides,each of said rods slotted along portions of its inner surface adjacentto portions of the helix extending within the two wave guides, and meanswithin each wave guide for matching the helix impedance to the waveguide impedance comprising a conductive ring member closely surroundingbut apart from the helix passing through the slots in the dielectricrods, and a field confining conductive element within each wave guidesurrounding the helix and the dielectric rods.

5. In an electronic device, a helical conductor, a hollow wave guideapertured in a side wall for admittance of one end of said helicalconductor, a conductive post extending from a side wall of said waveguide to which is connected said end of the helical conductor, aplurality of dielectric rods spaced apart around and in contact withthehelical conductor for its support within the wave guide, each of saidrods having a portion of its inner surface slotted, and a conductiveannular member passing through the slots in the dielectric rods andclosely surrounding a portion of the helical conductor in the waveguide.

6. An electronic device according to claim 5 which includes an aperturedconductive field confining element in the wave guide passing around thedielectric rods for surrounding a portion of the helical conductor andin contact with said conductive annular member.

7. In an electronic device, a helical conductor, a hollow wave guideapertured in a side wall for admittance of 7 one end of said helicalconductor, a conductive post extending into said wave guide to which isconnected said end of the helical conductor, a plurality of dielectricsupport rods spaced around and in contact with the outer surface of thehelical conductor, each of said support rods being spaced apart along aportion of its inner surface from said helical conductor for formingslots therebetween, and a conductive member in said wave guidecomprising an annular member passing through said slots for closelysurrounding a portion of the helical conductor.

8. In an electronic device according to claim 7 which further includesin the hollow wave guide an apertured element surrounding the dielectricrods and at the potential of said annular member.

9. In an electronic device, a helix, a hollow wave guide apertured in aside wall for admittance of one end of said helix, a conductive postextending into said wave guide to which is connected said end of thehelix, the portion of the end of said conductive post to which theconnection is made being beveled to the pitch of the helix, conductivemeans for confining the coupling between the helix and the wave guide toan end portion of the helix, a plurality'of dielectric support rodsspaced around and in contact with the outer surface of the helix, eachof said support rods being spaced apart along a portion of its innersurface from the portion of the helix following said coupling endportion for forming slots therebetween, and a conductive member disposedto pass through said slots for surrounding the portion of the helixfollowing said coupling end portion.

10. In a transmission system, a helix, a plurality of dielectric supportmembers extending along in contact with the helix surface, each of saidmembers slotted along a portion of their inner surface for providingthere along a separation from said helix, and means in an energyexchange relation with said helix including an outer hollow membercomprising a portion of the outer conductor of a coaxial terminal andhaving a conductive projection with an aperture therein which includesan annular element for closely surrounding the helix passing through theseparation formed between the helix and support members, and an innermember comprising a portion of the inner conductor of said coaxialterminal, one end of the helix being connected to said inner member.

11. In a wave transmission system, a helix, a wave guiding structure inenergy exchange relation with said helix including an outer hollowmember comprising a portion of the outer conductor of a coaxial terminaland an inner member comprising a portion of the inner conductor of saidcoaxial terminal, the helix being connected at one end to said innermember, a lateral member extending from said outer member towards saidinner member apertured for passage of the helix therethrough for formingtherebetween a coupling gap in which extends an end portion of thehelix, a plurality of dielectric support rods spaced around and incontact with the major portion of the outer surface of the helix butspaced apart from a portion of the helix following the coupling endportion for forming slots therebetween, and a conductive member passingthrough said slots for surrounding a portion of said helix.

12. In electronic apparatus, a helix, a wave guiding structure in energyexchange relation with said helix including an outer hollow membercomprising a portion of the outer conductor of a coaxial terminal and aninner member comprising a portion of the inner conductor of said coaxialterminal, the helix being connected at one end to said inner member, aplurality of dielectric support rods spaced apart around the helix, eachnotched, and a lateral member extending from the outer conductor of thecoaxial terminal comprising a ring member passing through the notches inthe dielectric rods for closely surrounding a portion of the helixfollowing the connected end of the helix for transforming the heliximpedance and a field confining member passing around the dielectricrods.

References Cited in the file of this patent A Broadband Transition fromCoaxial Line to Helix, RCA Review, vol. 11, No. 1, pages 133142, March1950.

